1. Field of the Invention
The present invention relates to a composition and a solution for forming a transparent conducting film and a method of forming a transparent conducting film using the solution, the transparent conducting film being useful as a transparent electrode in a display element of a liquid crystal display, plasma display, electroluminescence (EL), etc., a transparent electrode for a solar battery, a heating resistor for preventing fogging or frosting (freezing) in a window glass in front of a driver's seat of a vehicle or a pilot's seat of aircraft or in window panes of a building, or as an antistatic film, electromagnetic wave shielding film, infrared reflecting film, selectively light transmitting film and the like.
2. Description of Related Art
Use is made of materials which have a high transmittance for visible light and conductivity in forming transparent electrodes in display elements of liquid crystal, plasma, electroluminescence (EL) displays, etc., transparent electrodes for solar batteries, heating resistors for preventing fogging or frosting (freezing) in window glass of driver's seats of vehicles or pilot's seats of aircraft or in window panes of buildings, or in antistatic films, electromagnetic wave shielding films, infrared reflecting films, selectively light transmitting films and the like.
As such transparent conductive materials, tin oxide/antimony oxide(ATO) materials, indium oxide/tin oxide(ITO) materials and so on are known. The transparent conducting films made from these metal oxides are usually formed on glass or ceramic substrates. Among them, ITO (indium tin oxide) films find broadest application because of their high conductivity and transmittance.
Known methods of forming transparent conducting films include CVD methods (plasma CVD method and light CVD method), PVD methods (vacuum evaporation method, ion plating method and sputtering method) and coating method.
However, of the foregoing methods of forming transparent conducting films, CVD and PVD methods require large-scale equipment such as vacuum equipment. Furthermore, these methods require operation in a vacuum or inert atmosphere, and entail controlling the operational atmosphere. The CVD and PVD methods need to be improved in the low operational efficiency, high costs and unsuitability for mass production.
On the other hand, the coating method can easily form films in large quantities without the need for large-scale equipment, but involve defects depending on the film-forming materials as detailed below.
When using an organic acid indium having a high ionic bonding character such as indium octylate, the organic acid indium is likely to hydrolyze and relatively easily undergoes chemical change so that gelation occurs during preparation of the coating solution or during coating deposition. When the coating solution gels, the uniformity of obtained films is lowered and the conductivity and transmittance thereof are reduced.
In the method by which a coating solution containing a metal alkoxide as its main component is applied and an ITO film is formed by a sol-gel process, the metal alkoxide solution to be used as a coating solution usually contains a metal alkoxide such as methoxide, ethoxide or isopropoxide of indium or tin, and optionally additives for stabilization. Since metal alkoxides easily hydrolyze, the synthesis and subsequent handling of the metal alkoxide must be done in an inert atmosphere, resulting in a poor operational efficiency. When a metal alkoxide solution is applied to the substrate and is left to stand thereon for a long time, or the humidity of the operational atmosphere is high, the uniformity of the film is impaired because the metal alkoxide easily hydrolyzes, consequently resulting in difficulty in forming a conducting film of low resistance.
Journal of the Ceramic Society of Japan 102 [2]200–205 (1994) discloses an example of formation of an ITO film from indium nitrate trihydrate, anhydrous tin(IV) chloride, polyvinyl alcohol and water by a sol-gel process. According to the disclosed method, colloidal particles containing indium and tin are precipitated and separated by centrifugation. The particles are then ultrasonically dispersed in an aqueous solution of indium chloride or hydrochloric acid to form a sol. Thereafter polyvinyl alcohol is added to the sol as a film-forming aid, and the mixture is deposited on the substrate. Subsequently the coating film is dried to form a gel film. The gel film is fired at 550° C. to give an ITO film.
However, this method requires such complicated steps as precipitation of colloidal particles, centrifugation, ultrasonic dispersion, drying and firing.
Japanese Unexamined Patent Publication No. 2002-175733 describes a coating solution for forming an ITO transparent conducting film, the coating solution being prepared by adding a surfactant to a solution to improve the compatibility of the solution with the substrate, the solution comprising indium chloride and tin(II) chloride or tin(IV) chloride dissolved in water, alcohol or a mixture of water and alcohol. The publication further discloses a method wherein a thin film having a thickness of 15 nm or less is formed by application of the solution in one step by a dip coating method, whereby the unreacted residue in the film is decreased resulting in enhanced film quality.
However, a coating solution containing water has a markedly high surface tension, so that an improvement in wettability between the coating solution and the substrate by addition of the surfactant can not be expected. In particular it is presumed that numerous broken parts such as partially formed pinholes would tend to occur in the thin film formed by application of the solution in one step as described above. A multilayer film may be produced by laminating layers to repair the broken parts in the film, resulting in increased conductivity, but with lower operational efficiency and reduced mass-productivity. Furthermore, thin films formed by application of the solution in one step have significantly lower transmittance despite their thinness. A multilayer film produced by laminating such thin films is likely to have further lowered transmittance because light is entrapped between the layer interfaces. Therefore, it is difficult to obtain an ITO film having both good conductivity and a high transmittance using the coating solution and the method stated in Japanese Unexamined Patent Publication No. 2002-175733.
Japanese Unexamined Patent Publication No. 1994-175144 discloses a method using alkyl indium and alkyl tin. Generally alkyl metal compounds are significantly unstable and include, for example compounds which spontaneously ignite in air at room temperature such as triethyl indium.
A method using an organic complex of indium or tin was also proposed. However, the method necessitates use of an organic solvent at a high ratio to prepare a coating solution from an organic complex. Therefore, the solution is not preferrable from the viewpoint of labor hygiene, fire prevention and the global environment.
Further, when a transparent conducting film is used as the transparent electrode of a display device, patterning of the transparent electrode is reqired. Patterning is generally carried out by a method comprising complicated photolithographic (circuit patterning) step using photosensitive resist, etching, resist separation and the like. The ink jet printing method which forms a circuit pattern directly on the substrate is available as a simpler method. However this method is unable to use commercially available ink jet printing devices equipped with ink cartridges and other parts which are corroded by contact with organic solvents.